A Bomb Built in Hell — Andrew H. Vachss

While doing extended time for killing a fellow prisoner, Wesley meets Carmine Trentoni in a New York state prison. Carmine's life sentence hasn't cut him off from his outside sources, and he sees great potential in Wesley to carry out his revenge. Wesley emerges from prison prepared to be the perfect hitman: calculating, deadly, and driven by money. On his release, Wesley follows Carmine's directions to locate a Mr. Petraglia. Pet and Wesley set up shop in Brooklyn, and execute their assignments with finesse - and occasionally, more explosives than are strictly called for. But Wesley isn't satisfied with his low-profile lot, and sets out to make a mark on the city that everyone will notice - which he does, in a shocking, dynamite conclusion.